The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A RF power system may include a RF power supply (or RF generator), a matching network and a load. Guided search techniques may be used for impedance tuning the RF power supply and/or the matching network. Impedance tuning is performed to match, for example, an input impedance of the matching network to an impedance of a transmission line between the RF power supply and the matching network. This impedance matching maximizes an amount of power forwarded to the matching network and minimizes the amount of power reflected back from the matching network to the RF power supply.
An example method for impedance tuning includes coarse and fine frequency tuning the RF power supply. An objective of a frequency tuning algorithm is to determine a frequency such that a magnitude of a reflection coefficient of the RF power supply is at a minimum value. The smaller the reflection coefficient, the less power that is reflected back to the RF power supply. FIG. 1 illustrates reflection coefficient responses based on coarse and fine tuning of the RF power supply. In FIG. 1 a reflection versus frequency curve is shown. Coarse frequency hops are indicated by arrows 1-5 and fine frequency hops are indicated by arrows 6-8. For an operating frequency range FMax-FMin of the RF power supply, a minimum magnitude of the reflection coefficient is at a tune frequency FTune. The tune frequency FTune is located between two approximately flat (approximately zero slope) regions of the reflection coefficient versus frequency curve. The flat regions may have a reflection coefficient value of one.
Typically, a frequency tuning algorithm may include a heuristic technique to adjust the frequency to the tune frequency FTune. The heuristic technique commences with a first course frequency hop as indicated by arrow 1. The first coarse frequency hop may be performed in either direction. Based on the resulting reflection coefficient, a next coarse frequency hop is performed. Since the first frequency hop decreases the frequency of the RF power, increases the reflection coefficient, and increases the amount of reflected power, the first coarse frequency hop is not in the correct direction (i.e. towards FTune). The guided-search method continues with a determination that the decrease in frequency was inappropriate and the next course frequency hop is performed to increase the frequency of the RF power supply. This can return the RF power supply to an initial condition. As a result, multiple frequency hops are performed, which decreases RF power efficiency (ideally, reverse power is zero and all of the RF power is applied to the load) and increases tuning time. For this reason, a frequency tuning algorithm may be enhanced with an initial predetermined direction of a frequency hop to provide a more efficient path toward the minimum reflection coefficient.
The guided-search method produces subsequent course frequency hops that increase the frequency of the RF power supply. The action of increasing the frequency causes both the magnitude of the reflection coefficient and reverses power to decrease. The guided-search method continues to increase the frequency with course updates until a predetermined tune threshold is passed. When the guided-search method passes the predetermined tune threshold, the next frequency hop is in a reverse direction and is a fine frequency hop to proceed toward the tune frequency FTune. A result of driving the frequency past the tune frequency FTune there is an increase in the magnitude of the reflection coefficient and an increase in reverse power, which decreases RF power efficiency and increases tuning time. The guided-search method may continue to reverse the direction and size of frequency hops until the predetermined tune threshold is met. The frequency tuning may require multiple passes of the predetermined tune threshold and/or the tune frequency FTune before being completed.
FIG. 2 provides an example illustrating effects on reverse power associated with coarse and fine frequency tuning adjustments. In FIG. 2 a reverse power versus tuning time curve is shown. Coarse frequency hops are indicated by arrows 9-13. Fine frequency hops are indicated by arrows 14-16. FIG. 2 illustrates multiple passes of the tune frequency FTune before completing tuning. The reverse power may not be at a minimum level as shown in FIG. 2 when the frequency tuning is completed. This is because the tuning is completed when the predetermined tune threshold is no longer passed and/or is met, which may not result in the frequency of the power amplifier being at the tune frequency FTune.